You’ve probably heard about El Niño, the climate system that brings dry and often hotter weather to Australia over summer.

You might also know that climate change is likely to intensify drought conditions, which is one of the reasons climate scientists keep talking about the desperate need to reduce greenhouse gas emissions, and the damaging consequences if we don’t.

El Niño is driven by changes in the Pacific Ocean, and shifts around with its opposite, La Niña, every 2-7 years, in a cycle known as the El Niño Southern Oscillation or ENSO.

But that’s only part of the story. There’s another important piece of nature’s puzzle in the Pacific Ocean that isn’t often discussed.

It’s called the Interdecadal Pacific Oscillation, or IPO, a name coined by a study which examined how Australia’s rainfall, temperature, river flow and crop yields changed over decades.

Since El Niño means “the boy” in Spanish, and La Niña “the girl”, we could call the warm phase of the IPO “El Tío” (the uncle) and the negative phase “La Tía” (the auntie).

These erratic relatives are hard to predict. El Tío and La Tía phases have been compared to a stumbling drunk. And honestly, can anyone predict what a drunk uncle will say at a family gathering?

What is El Tío?

Like ENSO, the IPO is related to the movement of warm water around the Pacific Ocean. Begrudgingly, it shifts its enormous backside around the great Pacific bathtub every 10-30 years, much longer than the 2-7 years of ENSO.

The IPO’s pattern is similar to ENSO, which has led climate scientists to think that the two are strongly linked. But the IPO operates on much longer timescales.

We don’t yet have conclusive knowledge of whether the IPO is a specific climate mechanism, and there is a strong school of thought which proposes that it is a combination of several different mechanisms in the ocean and the atmosphere.

Despite these mysteries, we know that the IPO had an influence on the global warming “hiatus” – the apparent slowdown in global temperature increases over the early 2000s.

Global temperatures are on the up, but the IPO affects the rate of warming.Author provided, data from NOAA, adapted from England et al. (2014) Nat. Clim. Change

Temperamental relatives

When it comes to global temperatures we know that our greenhouse gas emissions since the industrial revolution are the primary driver of the strong warming of the planet. But how do El Tío and La Tía affect our weather and climate from year to year and decade to decade?

Superimposed on top of the familiar long-term rise in global temperatures are some natural bumps in the road. When you’re hiking up a massive mountain, there are a few dips and hills along the way.

Several recent studies have shown that the IPO phases, El Tío and La Tía, have a temporary warming and cooling influence on the planet.

In the negative phase of the IPO (La Tía) the surface temperatures of the Pacific Ocean are cooler than usual near the equator and warmer than usual away from the equator.

Since about the year 2000, some of the excess heat trapped by greenhouse gases has been getting buried in the deep Pacific Ocean, leading to a slowdown in global warming over about the last 15 years. It appears as though we have a kind auntie, La Tía perhaps, who has been cushioning the blow of global warming. For the time being, anyway.

The flip side of our kind auntie is our bad-tempered uncle, El Tío. He is partly responsible for periods of accelerated warming, like the period from the late 1970s to the late 1990s.

The IPO has been in its “kind auntie” phase for well over a decade now. But the IPO could be about to flip over to El Tío. If that happens, it is not good news for global temperatures – they will accelerate upwards.

Models getting better

One of the challenges to climate science is to understand how the next decade, and the next couple of decades, will unravel. The people who look after our water and our environment want to know things like how fast our planet will warm in the next 10 years, and whether we will have major droughts and floods.

To do this we can use computer models of Earth’s climate. In our recently published paper in Environmental Research Letters, we evaluated how well a large number of models from around the world simulate the IPO. We found that the models do surprisingly well on some points, but don’t quite simulate the same degree of slow movement (the stubborn behaviour) of El Tío and La Tía that we observe in the real world.

But some climate models are better at simulating El Tío and La Tía. This is useful because it points the way to better models that could be used to understand the next few decades of El Tío, La Tía and climate change.

Concerns about drought and water supply are once more building in eastern Australia. Recent reports from Victoria show the state government is considering switching on the so-far-unused desalination plant to supply Melbourne. While Melbourne doesn’t currently need extra water, this might free up other water allocated to the city to be diverted to regional communities in the north of the state where water shortages are looming.

When drought strikes, people and governments look to shore up water supplies. In Australia, politicians have focused on building more dams and long pipelines, at the expense of alternative sources such as recycled water.

It has been widely assumed that drinking recycled water, from sources such as sewage, is not acceptable to the public. But an Australia-first survey released by the Australian Water Association shows the public is ready to accept recycled water.

Water is getting further away and more expensive

Since colonisation, we have tapped increasingly distant, more energy-intensive and more expensive sources of fresh water.

We have constructed large dams to buffer variable water supplies through wet and dry seasons, as well as wet and dry years. As water consumption has exceeded the capacity of river basins to meet demand, we have constructed long pipelines, to pump water in from less populated river basins to more populated basins.

During the last decade, many of our major cities began to identify that the capacity to pump more water from distant locations was approaching sustainable limits.

New sources of water were required. So, in a very short period of time, cities including Perth, Sydney, Melbourne, Adelaide and the Gold Coast set about constructing seawater desalination plants.

Hindsight comes with 20-20 vision and it’s clear that most of these desalination decisions – with the notable exception of Perth’s – were premature since the plants have barely beeen used. Politicians and those with a financial interest in these plants often comment that the desal plants represent great “insurance policies” for when the next drought inevitably arrives.

However, these “insurance policies” came with billion-dollar price tags. Much of this has been externally financed, thus accruing significant annual interest costs.

Furthermore, the costs associated with maintaining desal plants are significant – even when they supply zero or negligible water. In some cases, such as Sydney, the real need to use the desal plant – given effective demand management – is likely to be still many years away. As such, it is arguable that these desal plants were very poor value insurance policies.

I argued that the technology was established to reliably purify water from sewage treatment plants to such a high degree that it would be capable of providing extremely high-quality drinking water for Sydney. This practice has been adopted in a number of US cities and is commonly referred to as “potable water recycling”.

All towns and cities are physically unique in terms of geography and historic development features. However, in the right mix of circumstances, potable water recycling can have significant advantages over seawater desalination.

These can include reduced operation and construction costs, as well as much lower energy requirements, which translate to reduced carbon emissions. Nonetheless, the suggestion that the NSW government seriously consider potable water recycling as an alternative to seawater desalination was not widely appreciated.

The general wisdom of the time was that Australians would not be prepared to accept water that was once sewage as a component of their drinking water supply. Indeed, this appeared to be supported by a telephone survey around that time.

Since then, the New South Wales and Victorian state governments have made statements that potable water recycling is not even an option for consideration by cities in those states.

Political thirst for dams

It is widely recognised that most opportunities for building dams on rivers to provide water for Australia’s large cities have been effectively exhausted.

Nonetheless, Australian politicians appear to yearn for opportunities to announce a new big dam project. When the federal member for Calare, John Cobb, announced a plan to dam the Belubula River at Needles Gap (NSW) in 2014, he declared: “I believe this project will lift the spirits of the central west and will inspire all of regional Australia.”

Water supply projects may have many diverse objectives, and inspiring all of regional Australia may be an understandably important one for a politician. However, many politicians appear to carry some unshakeable assumptions about community water supply preferences in Australia. Most seem to think we all want to hear announcements for new big dams.

And if we can’t have new big dams, they think desalination plants are our next preferred option. Few politicians deny the sustainability advantages of potable water recycling, but most seem to think it’s just too difficult to bring the community on board to support it.

Attitudes are changing

In the recent survey, 3,316 completed responses were received from community members across Australia.

Of these, 69% agreed or strongly agreed with the statement that recycled water “can be treated and managed for safe drinking”. This compared with 56% who agreed with the same statement for stormwater and 82% who agreed for seawater desalination.

Given the prevalence of actual seawater desalination plants around Australia and a lack of any public discussion about potable water recycling, I suggest that this level of faith in the capabilities of recycling plants is remarkable.

Recent research from the United States has shown that by engaging the community and providing accurate information, the underlying level of support for recycling can be significantly increased.

When consumers were asked whether they agree with the statement that “there is scope for more dams to provide additional water supplies in the south of Australia (e.g. in the Murray-Darling Basin and the south-east coastal areas)”, only 33% agreed. This rose to 46% for northern Australia. So much for inspiring all of regional Australia.

When this happens, we can expect many regional areas to struggle in their management of dwindling water supplies. Many will be searching for sustainable water supply solutions and some will identify potable water recycling as the most sustainable option for their circumstances.

The challenge for the federal and state governments will be to support the needs of these towns and cities. They will do that best by ensuring that all potential water supply options are on the table and given fair consideration.

In the meantime, our politicians would serve regional Australia best by ceasing to stigmatise potable water recycling as an option that is not even entitled to consideration.

Instead, they should work to build upon the support that currently exists in our communities so that when the need arises, potable water recycling is a viable and broadly accepted water supply solution. There is already powerful evidence that this can be achieved, when state governments work constructively toward this goal.

In 2013, the Western Australian government gave strong support and approval for a potable water recycling project to provide up to 20% of Perth’s water supply. That plant has since been constructed and will soon begin replenishing one of the city’s essential, but dwindling, groundwater supplies.

The anticipation is growing that this year’s newly formed El Niño will turn out to be very big. All climate models surveyed by the Australian Bureau of Meteorology are currently predicting a strong event later this year.

We’ve been here before – last year, in fact, when fears of a 2014 “super El Niño” proved anticlimactic. But it’s not over yet. The El Niño – Spanish for “the little boy”, which refers to a particular pattern of ocean and atmospheric temperatures across the Pacific – has resumed its growth this year and this time it is not showing any signs of slowing down.

It’s easy to see why this little boy gets so much attention. First, we are talking about a climate phenomenon that brings drought, rains, floods, heatwaves and other extreme weather events to many parts of the world.

Second, it is almost 20 years since the previous extreme El Niño. The 1997-98 event was the biggest in modern records and its worldwide catastrophic impacts earned it the infamous description of “the climate event of the 20th century”. A comparable but slightly weaker El Niño occurred in the summer of 1982-83, which was marked by severe drought in eastern Australia and the tragic Ash Wednesday bushfires.

Third, the latest climate model projections – reviewed by us in a study published today in Nature Climate Change – have shown that Earth will probably experience more super El Niños as the global climate warms. The projections also suggest that the extreme version of La Niña – the sister and “opposite” of El Niño – will also increase in frequency, as will the positive phase of the siblings’
“cousin”, a related phenomenon called the Indian Ocean Dipole. This also includes more successive occurrences of the trio.

A family gathering

This atypical “family gathering” has happened before. A positive Indian Ocean Dipole occurred in the southern spring of 1997, before the El Niño peaked the following summer. A La Niña then followed in the summer of 1998-99. For western Pacific rim countries, the overall result was drier-than-normal conditions in 1997, followed by unusually wet conditions in 1998. A similar series of events also occurred in 1982-83.

To understand this and to see how global warming spurs such events, we need to understand the physics of El Niño, taking the most recent unfolding events as a start.

The 2014-2016 chain of events would be interesting in its own right. While the failed 2014 super El Niño left many experts scrabbling for an explanation, its warming remnants in the central Pacific have now transformed into the official 2015 El Niño.

It is not common that two El Niño events would occur consecutively. The heat accumulated in the equatorial Pacific Ocean that fuels an El Niño is usually discharged, and some of this heat goes into the atmosphere, where it helps to accelerate warming in global surface temperature.

The discharge is proportional to the intensity of an El Niño. So the stronger the El Niño, the stronger the discharge. Also, the stronger the El Niño, the more dramatic the weakening of the Walker Circulation, with slackened trade winds and equatorial currents. This weakening Walker Circulation extends into the Indian Ocean which tends to induce a positive Indian Ocean Dipole during the southern spring.

Following the peak of an El Niño in the southern summer, the equatorial Pacific Ocean is depleted of heat and needs to be recharged. Winds associated with La Niña are effective in this recharge, and so an El Niño tends to be followed immediately by a La Niña.

Clearly the 2014 El Niño conditions were not strong enough for this to happen. Another similar exception occurred during 1986-1988. The 1986-87 El Niño was weak, the Pacific Ocean heat was not completely depleted, allowing for another somewhat stronger El Niño in 1987-88. These two events are considered weak to moderate.

However, after the two consecutive events, the equatorial Pacific was finally depleted of heat. The subsurface ocean was colder, facilitating surface cooling in the central Pacific through a suite of atmosphere-ocean positive feedback processes, leading to the extreme La Niña of 1988-89.

What is in store?

In terms of intensity and the growth rate up to July, the 2015 El Niño is second only to corresponding time of the 1997 event, and looks set to outpower the 1982 event. However, the eventual intensity of the 2015 El Niño is still hard to predict. What seems more certain is a La Niña in 2016.

For Australia, the extent and strength of the impact of the 2015 El Niño to a large extent depends on whether there is a concurrent positive Indian Ocean Dipole. In 2014, there was no positive Indian Ocean Dipole. To date, most models are predicting a positive dipole this year, raising the prospect of a strong El Niño preceded by a positive Indian Ocean Dipole and followed by a La Niña event – exactly as occurred in 1982-84 and 1997-99.

The pattern of Pacific Ocean temperatures during the last strong El Nino event in 1997.NOAA

For Australia, the impacts of this sequence could be significant, as attested by the devastating Ash Wednesday bushfire in 1983 over southern Australia and the floods that hit the country’s northeast in early 1984.

This swing between opposite extremes from one year to the next could have globally damaging consequences too. On the far side of the Pacific, California may get a break from its a prolonged drought, although this hopefully won’t be in the form of intense storms and flooding.

Climate change bringing extremes

We cannot be sure if climate change plays a role in an individual event, and climate models are certainly not perfect. Observations need to be sustained to gather and compare robust statistics. But due to recent research we can now say that stronger El Ninos, La Ninas, and positive Indian Ocean Dipoles are all to be expected on a warming planet.

Climate models project an overall weakening of the Walker Circulation over the 21st century, underpinned by faster warming in the eastern equatorial Pacific (which is favourable for extreme El Niños, and in turn conducive to extreme La Niña). There will also be faster warming in the western than the eastern Indian Ocean, which would tend to promote positive Indian Ocean Dipole events. As a consequence, the sequence of an El Niño preceded by a positive Indian Ocean Dipole and followed by a La Niña event is projected to occur more frequently.

These sequences of events are likely to affect a vast swathe of the planet, extending from Africa, right through to South Asia and Australasia, and all the way across to the coastlines of the eastern Pacific.